1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a thin film transistor liquid crystal display (TFT-LCD) device implementing multi-domains for a liquid crystal.
2. Discussion of the Related Art
Recently, liquid crystal display (LCD) devices with light, thin, low consumption characteristics are used in office automation equipment, video units and the like. Among various type devices, thin film transistor liquid crystal display (TFT-LCD) devices are widely used because of their superior color-displaying quality and advantage of thickness.
As display areas of liquid crystal display devices are made larger and larger, the quality of the viewing angle of the LCD devices becomes the more important property among various quality factors of the liquid crystal display device. To improve the quality of the viewing angle, additional retardation films or diffusion layers have been used in liquid crystal panels of the liquid crystal display devices. And further, instead of these expensive improved methods, a method of aligning the liquid crystal in different orientations was developed.
That is to say, a plurality of different electric fields parallel with a substrate are adapted to align the liquid crystal molecules in various domains. To achieve the differently directed electric fields parallel with the substrate, common and pixel electrodes are formed to have different areas.
In detail, a first portion of the liquid crystal in a first electric field is aligned in a first orientation, while a second portion of the liquid crystal in a second electric field is aligned in a second orientation such that first and second domains of the liquid crystal are defined. Since molecules in the first domain have a different orientation from that of molecules in the second domain, the viewing angle of the liquid crystal is widened.
FIG. 1 shows the configuration of a typical TFT-LCD device. The TFT-LCD device 1 includes upper and lower substrates 10 and 20 with a liquid crystal 50 interposed therebetween. The upper and lower substrates 10 and 20 are called a color filter substrate and an array substrate, respectively.
In the upper substrate 10, on a surface opposing the lower substrate 20, black matrix 12 and color filter layer 14 that includes a plurality of red (R), green (G), and blue (B) color filters are formed in an array matrix such that each color filter is surrounded by the black matrix 12. Further on the upper substrate 10, a common electrode 16 is formed and covers the color filter layer 14 and the black matrix 12.
In the lower substrate 20, on a surface opposing the upper substrate 10, a TFT “T”, as a switching device, is formed in an array matrix corresponding to the color filter layer 14, and a plurality of crossing gate and data lines 26 and 28 are positioned such that each TFT is located near each intersection of the gate and data lines 26 and 28. Further, in the lower substrate 20, a plurality of pixel electrodes 22 are formed on an area defined by the gate and data lines 26 and 28, a pixel portion “P”. The pixel electrode 22 is a transparent conductive metal such as indium tin oxide (ITO).
To align the liquid crystal 50 in different orientations for improving the viewing angle, the structure around the pixel portion P is conventionally formed, as shown in FIGS. 2 and 3. On the pixel portion P, a side electrode 30 surrounds the pixel electrode 22 in a position slightly below the pixel electrode 22, as shown in FIG. 3. The side electrode 30 is electrically connected with the common electrode 16 formed on the upper substrate 10 of FIG. 1.
As shown in FIG. 3, the pixel electrode 22 is spaced apart from the common electrode 16. A gap or slit 18 forms a through hole in the common electrode. The slit 18 is formed in a position corresponding to a longitudinal center of the pixel electrode 22, as indicated by longitudinal center line 32. Because the side electrode 30 is lower than the pixel electrode 22 and electrically connected with the common electrode 16, when there is a voltage difference between the pixel and common electrodes 22 and 16, different electric fields 34a and 34b are induced, respectively, in the liquid crystal 50 in first and second domains “A” and “B”. First and second domains “A” and “B” are separated by a boundary domain “C”, and centered on the slit 18. The first and second electric fields 34a and 34b in the first and second domains A and B are tilted outward to the side electrode 30, and little or no electric field exists in the boundary domain “C”.
Since the liquid crystal 50 is aligned in different orientations in the multi-domains including the first and second domains A and B, the viewing angle quality of the LCD device is improved.
FIGS. 4 and 5 show another conventional liquid crystal display device similar to FIGS. 2 and 3. In the structure shown in FIGS. 4 and 5, an organic rib 19 substitutes for the slit 18 of FIGS. 2 and 3. The first and second domains A and B and a boundary domain C are defined by the rib 19. Similarly to FIG. 3, the first and second electric fields 34a and 34b define the multi-domains, i.e., the first and second domains A and B.
However, in the above-mentioned conventional liquid crystal display devices implementing the multi-domain liquid crystal, the side electrode is opaque and decreases the area of the pixel electrode. Thus, the aperture ratio is much lower than that of a mono-domain liquid crystal display device. The actual aperture ratio of the above-mentioned liquid crystal display devices are about 45%, while that of the typical mono-domain liquid crystal display device implementing the mono-domain with twisted neumatic liquid crystal (TN-LC) is about 65%. The decrease in aperture ratio results in decrease in brightness by about 30%.
Further, an additional fabrication step of photolithography is added to form the slit or rib on the common electrode, so that fabricating processes become much complicated.